1. Field of the Invention
The present invention is in the field of integrated optics, and, in particular, is in the field of integrated optical wavelengths formed by diffusing a material into a substrate to change the refractive index of a portion of the substrate to form the waveguide.
2. Description of the Related Art
Integrated optical waveguides are well-known in the art. Such waveguides are formed, for example, by diffusing a dopant material into a substrate such that a portion of the substrate comprises a diffused layer that has different light propagation characteristics than the original substrate. By controlling the depth and concentration of the diffused layer, a waveguide having desired optical propagation characteristics can be obtained. Prior integrated optical waveguides have been formed by diffusing titanium (Ti) into electro-optic materials such as lithium niobate (LiNbO.sub.3) and LiTaO.sub.3, as illustrated, for example, in G. L. Tangonan, et al., "High optical power capabilities of Ti-diffused LiTaO.sub.3 waveguide modulator structures," Applied Physics Letters, Vol. 30, No. 5, Mar. 1, 1977, pp. 238-239. Integrated optical waveguides have also been formed by vapor diffusion of zinc (Zn) into LiTaO.sub.3, as illustrated, for example, in D. W. Yoon, et al., "Characterization of Vapor Diffused Zn:LiTaO.sub.3 Optical Waveguides," JOURNAL OF LIGHTWAVE TECHNOLOGY, Vol. 6, No. 6, June 1988, pp. 877-880. Other integrated optical waveguides have been formed by proton exchange, as illustrated, for example, in P. G. Suchoski, et al., "Stable low-loss proton-exchanged LiNbO.sub.3 waveguide devices with no electro-optic degradation," OPTICS LETTERS, Vol. 13, No. 11, November 1988, pp. 1050-1052; J. J. Veselka, et al., "LOW-INSERTION-LOSS CHANNEL WAVEGUIDES IN LiNbO.sub.3 FABRICATED BY PROTON EXCHANGE," ELECTRONICS LETTERS, Vol. 23, No. 6, Mar. 12, 1987, pp. 265-266; and J. Jackel, et al., "Damage-resistant LiNbO.sub.3 waveguides," Journal of Applied Physics, Vol. 55, No. 1, Jan. 1, 1984, pp. 269-270. There have also been combinations of the diffusion and proton exchange techniques integrated optical components in order to obtain characteristics from both processes, as illustrated for example, in F. J. Leonberger, et al., "LiNbO.sub.3 and LiTaO.sub.3 Integrated Optic Components for Fiber Optic Sensors," Optical Fiber Sensors, Proceedings of the 6th International Conference, OFS'89, Paris, France, Sept. 18-20, 1989, pp. 5-9; P. G. Suchoski, et al., "Low-loss high-extinction polarizers fabricated in LiNbO.sub.3 by proton exchange," OPTICS LETTERS, Vol. 13, No. 2, February 1988, pp. 172-174; and T. Findakly, et al., "SINGLE-MODE TRANSMISSION SELECTIVE INTEGRATED-OPTICAL POLARISERS IN LiNbO.sub.3," ELECTRONICS LETTERS, Vol. 20, No. 3, Feb. 2, 1984, pp. 128-129.
As discussed in the above-cited references, the different processes result in different characteristics for the waveguides manufactured in accordance with the processes. For example, titanium-diffused waveguides have adequate performance for some applications, such as propagating light in the infrared portion of the spectrum, but are notoriously inadequate for operation in the visible portion of the spectrum because of high photorefractive sensitivity which causes scattering. Waveguides formed using proton exchange techniques provide better performance in the visible portions of the spectrum. However, the proton exchange process is a relatively low-temperature process (an exchanging temperature of approximately 200.degree. C.-250.degree. C.) comparted to the titanium-diffusion process (an annealing temperature of approximately 900.degree. C.-1100.degree. C.). It is believed that the higher temperature process results in a more stable waveguide since the operating temperature of the waveguide will be much further away from an annealing temperature at which the waveguide was formed. In addition, a zinc waveguide will guide both polarizations instead of only one polarization as in a waveguide formed using a proton exchange process.
As set forth above, zinc has been used as a diffusion material on LiTaO.sub.3 substrates with some success. However, when attempts were made to diffuse zinc on lithium niobate, severe pitting and surface damage were observed on the substrates. Thus, zinc has been considered to be an unsuitable diffusion material for lithium niobate.
There has existed a need for a material that can be used in combination with a lithium niobate substrate to provide an optical waveguide that can be used with visible light, is manufactured with a high temperature process, and which has suitable optical propagation characteristics such as guiding both polarizations.